    1. U.S. Pat. No. 7,054,226 May 30, 2006 issued to R. Hickling entitled “Method and Apparatus for Echolocation”,discloses the use of the vector sound-intensity probe and ultrasound in air for applications such as robot guidance, aids to the blind, and simulating bat sonar. Use of the vector sound-intensity probe is an important part of the present invention. Sound intensity is sound-power flow per unit area. There are two types of vector sound-intensity probe: a full-space probe that measures vector sound intensity in three dimensions, described in    2. R. Hickling, “Acoustic Measurement method and apparatus” U.S. Pat. No. 7,058,184 Jun. 6, 2006which is hereby incorporated herein by reference; and a half-space vector probe that measures sound intensity in the half space above the ground or next to a wall, described in    3. R. Hickling, “Vector sound-intensity probes operating in a half space”, patent application Ser. No. 11/607,376, Dec. 2, 2006.which is hereby incorporated herein by reference. The half-space probe is used in the present invention to measure vector sound intensity ahead of the boat or ship.
Vector probes in water have been shown to measure direction accurately, as demonstrated in    4. R. Hickling and W. Wei, “Use of Pitch Azimuth Plots in Determining the Direction of a Sound Source in Water with a Vector Sound-Intensity Probe”, Journ. Acoust. Soc. Amer., 97, 856-865. 1995.What is needed is a device that uses low frequency sound in water to enable ships and boats to avoid underwater obstacles. Low-frequency sound at about 30 kHz and below is required to penetrate sediment-laden water ahead of the ship or boat. This is particularly important in riverine and littoral environments. Fish finders, acoustic cameras and narrow-beam search devices operate at higher frequencies and are incapable of penetrating very far in sediment-laden water.